Whitefish Side-Stream Products as a Lower-Impact Alternative to Fishmeal in Atlantic Salmon Diets

In the past, trimmings, viscera, and heads from fisheries were often discarded as waste. However, these side-stream products, are recognized as valuable resources that can contribute to sustainable aquaculture. While the utilization of pelagic fish side-stream

products is well-established, side-stream products from whitefish remain underutilized. This study explores how these resources can partially replace high-quality fishmeal in salmon feed while maintaining growth performance and sustainability. 

A research collaboration between BioMar, Pelagia, Aker Biomarine, Nofima, and the University of Bergen investigated the potential of using side-stream products from filleted Atlantic cod (*Gadus morhua*) in salmon feed. The study, supported by the Norwegian Research Council, aimed to optimize fish feed formulations by evaluating the nutritional and physiological impacts of these novel ingredients. 

Four experimental feed ingredients were developed using different processing technologies, including fishmeal from heads and backbones (FM-hb), fish protein

hydrolysate from heads and backbones (FPH-hb), fishmeal from heads, backbones, and viscera (FM-hbg), and fish protein concentrate from viscera (FPC-g).

These ingredients were included in experimental diets, each replacing 50% of fishmeal protein. The study also included two control diets: a medium fishmeal control (FM10) and a low fishmeal control (FM5). The six diets were tested on post-smolt Atlantic salmon (*Salmo salar* L.) over an eight-week period to evaluate growth, feed intake, feed efficiency, and physiological responses. 

Results

The results demonstrated that all diets containing side-stream products performed well, with no significant differences in growth , feed intake (FI), feed conversion ratio (FCR), or body composition compared to the control diets. Even though not significant, the FM-hb and FM-hbg diets exhibited:

• Similar growth rates as FM10 

• 10% lower FCR 

• Higher protein efficiency ratio (PER) 

• Lower visceral fat levels 

These findings suggest that side-stream-based feeds can deliver comparable performance while enhancing sustainability. 

**Appetite and Digestion**  

A deeper examination of appetite regulation and digestion provided additional insights. The study focused on gene expression related

to the gut-brain axis, identifying potential biomarkers for appetite and digestion in salmon. 

• The gene *agrp1*, which stimulates appetite, was found at higher levels in hungry salmon and correlated with greater feed intake in fish receiving FM10. This suggests *agrp1* could serve as a biomarker for appetite in Atlantic salmon. 

• For hunger hormone ghrelin, the *ghrl1* gene variant was consistently more expressed than*ghrl2*, indicating its primary role in appetite regulation and energy metabolism in salmon. 

• For digestive hormones Pyy and Gcg the gene variants *pyya1*, *pyya2*, and *gcga* were significantly more expressed in the midgut compared to the hindgut, differing from mammalian digestion patterns. These hormones are believed to regulate digestion and satiety in fish and may serve as biomarkers for efficient digestion. 

The study highlights that whitefish side-stream products can effectively replace a portion of high-quality fishmeal in salmon feed without compromising growth or feed efficiency. Additionally, agrp1 may serve as a biomarker for appetite regulation, while pyya1, pyya2, and gcga could indicate rapid satiation feedback for more efficient digestion. 

By integrating these underutilized marine resources into fish feed, the aquaculture industry can reduce its reliance on traditional fishmeal sources, enhancing sustainability while maintaining performance.